Thin film magnetic heads comprise thin films providing a magnetic circuit, conductor coils, electrodes, etc. and formed in layers on a substrate with an insulating layer interposed between the adjacent layers. These heads are fabricated by thin film forming techniques such as vacuum evaporation, sputtering and photolithography and have the advantage that they can be compacted and given an increased density more easily than conventional bulk-type magnetic heads.
However, since the thin film magnetic head is much smaller than the bulk-type magnetic head in the depth of the magnetic gap, i.e., the dimension thereof (gap depth) in a direction perpendicular to the surface of the magnetic recording medium, the portion of the head to be opposed to the medium must be formed by lapping with high accuracy, for example of not greater than 1 .mu.m in error.
Accordingly, Examined Japanese Utility Model Publication SHO 62-35133 proposes, as seen in FIG. 12, a pair of markers 4, 4 in the form of a thin film and provided on the upper surface of a substrate 1 having magnetic cores 5, coil conductors 2, 3, etc. for use in detecting the amount of working performed to produce a magnetic head.
The two markers 4, 4 are each in the form of a rectangular equilateral triangle. The vertex of the triangle is positioned on a line A--A through the positions 5a, 5a of inner ends of the respective magnetic gaps 90. The base of the triangle intersects the direction of progress of working at right angles therewith.
When the workpiece has been lapped, for example, to the position of the line B--B shown in the process for producing the magnetic head, the current gap depth Dg can be calculated by measuring the widths Wa, Wb of the two marker films and utilizing the similarity of triangles of the markers. If there is a difference between the two measurements Wa and Wb, the difference indicates the angle of inclination of the worked surface with respect to the line A--A.
In producing the thin film magnetic head by the above process, the gap depth is calculated only from the measurements of the marker widths Wa, Wb, and the lapping operation is discontinued when a predetermined gap depth is obtained, so that when the triangle of the marker fails to retain its shape, for example, owing to chipping or plastic deformation during lapping, the proportional relationship between the gap depth and the measurements of the widths Wa, Wb is lost, producing a great error in the gap depth on completion of lapping.
Many markers may be provided to overcome this problem, but the magnetic head will then become large-sized.
On the other hand, Unexamined Japanese Patent Publication SHO 62-232718 proposes a thin film magnetic head wherein a pair of markers positioned one above the other are formed on the substrate and so adapted that the gap depth can be calculated by measuring the width of one of the marker films appearing on the lapped surface. Nevertheless, the proposal has the drawback that it is impossible to recognize the amount remaining to be lapped during lapping.